I've gone as much as 800 rounds of Dynapoints in my M261, 1:12 barreled rifle between .223 blowouts of the gas system. This time round I was going for 1000 rounds of Remington Golden Bullets, this particular barrel's favorite fodder. Didn't make all 1000 of GBs, as in testing the BDM hi-cap mags I had to use somewhere between 300 and 400 Federal Walmart Bulks, but I did make the 1000 rounds this morning.
Recently some pants-wetter published in a newsstand magazine that you risk blowing up your AR if you shoot .223s in it after .22LRs, because the bore gets "obstructed" with lead, raising chamber pressures drastically. I have NEVER heard of this actually happening (and you'd think someone here would have heard of it), but I took a look down the bore before my .223 blowout just to see. The bore looked dirty as you'd expect but the lands and grooves were very distinct. But just to be safe, I made ONE pass down the bore with a USGI bore brush. No giant chunks of lead came out.
I loaded an ancient GI mag with five rounds of Federal American Eagle and pulled the trigger. The first round ejected normally but the bolt seemed to short-stroke a bit, hitting the second round in a classic bolt-over-base failure that did not damage the round. Partially clogged gas systerm? Bad mag? Weak mag spring? Completely dry bolt? Who knows? After chambering it, the second round fired and ejected normally and the third round fed and chambered fully. I ejected it and put the other two rounds of .223 back in the box for another day. As far as I'm concerned, my gas system is now CLEAR (why waste 18 more rounds confirming it?) and the gun is good to go.
Next time I'll go 1200 or 1250 rounds before a blowout.